“Oasis” is on our list of banned travel words (along with “journey,” “wanderlust” and “hidden gem”), but if any place comes close to earning that tired description, it’s La Gruta in San Miguel de Allende, Mexico. Located about 20 minutes from Centro, the Grotto spa has multiple swimming pools and a domed cave fed by natural hot springs. Navigating the long rock-walled tunnel leading to the steamy cavern is good fun—you’ll feel like you’re in The Poseidon Adventure, minus the whole certain-death thing. Facilities are basic but clean and family-oriented; go early on a weekday to avoid squealing, splashing children. Entry is 110 MXN—less than $6.50 USD; aromatherapy massages cost a bit extra. BYO towels and padlock for the locker room.

Km 10, Dolores Hidalgo, San Miguel de Allende, Mexico; +52-415-185-2162.